1. Field of the Invention
The present invention relates to a differential hydraulic jack for the control of electric circuit-breakers, of the type in which the annular chamber of the jack as defined by the internal surface of the jack cylinder and by the external surface of the emergent piston-rod is continuously connected to a highpressure hydraulic fluid source.
2. Description of the Prior Art
The emergent rod of the hydraulic jack is coupled to the moving contact of the circuit-breaker and a supply/drain orifice formed in the bottom end of the main chamber of the jack can be selectively connected to said high-pressure source ("supply" position) so as to exert an outward thrust on the piston or else to a lowpressure tank ("drain" position) in order to allow the piston to return to its initial position under the action of the high pressure P maintained within the annular chamber.
The first operation causes outward displacement of the piston-rod and moves the circuit-breaker to the engaged or closed position whilst the second operation causes inward displacement of the piston-rod into the cylinder and moves the circuit-breaker to the disengaged or open position.
Hydraulic circuit-breaker control systems of the differential jack type considered in the foregoing are well-known and have been described for example in French patent No. 2,317,532 (or in U.S. Pat. No. 4,026,523).
The design of differential jacks for this application gives rise to constructional difficulties, in particular by reason of the fact that they have to guarantee permanent and absolute leak-tightness over very long periods of time despite very high hydraulic-fluid service pressures P of the order of 300 to 400 bar.
As shown in the prior patent cited earlier, these jacks must therefore be provided with a first packing seal at the point of emergence of the piston-rod through the bottom end of the cylinder and with a second packing seal on the piston.
This second packing seal has to withstand severe operating conditions and is therefore difficult to construct. If it is required to provide perfect leak-tightness, this seal should preferably be of the "spring-loaded packing" type.
In French patent Application No. 87.04.134 filed on Mar. 25th, 1987 in the name of the same inventor, it was shown that, in the differential jack described therein, the piston seal could be dispensed with by combining the piston with a check-valve for shutting-off the supply/drain orifice of the jack at the end of the tripping stroke of said jack.
In the construction of hydraulic jacks for the control of electric circuit-breakers, a further difficulty arises from the need to ensure highly effective damping of the ends of travel of the piston. In fact, since the movements of travel or strokes of the piston necessarily take place during a very short time interval of the order of a few hundredths of a second, operations are very abrupt and steps must accordingly be taken to slow-down or damp the motion of the piston at the ends of travel. This problem is aggravated by the fact that, in this application, only a very short distance (of the order of 20 to 50 mm) is available for the damping operation.
In known types of damping systems for hydraulic jacks, a floating ring is mounted in the end of the jack cylinder. Within this floating ring is engaged a substantially frusto-conical extension or damping stud carried by the piston. At the end of the piston stroke, the cross-sectional area of the annular passage located between the internal surface of the damping ring and the aforesaid extension of the piston progressively decreases, thus producing a progressive wiredrawing or throttling action on the oil contained in the jack chamber between the piston and the cylinder end which carries the damping ring. The oil-wiredrawing action just mentioned has the effect of damping the end of travel of the piston.
One example of a damping floating ring for a hydraulic jack is given in British patent No. 998,753 to Parker Hannifin and, more particularly for the specific application to the control of electric circuit-breakers, in French patent No. 2,317,532 cited earlier.
Known damping systems of the floating ring type provide satisfactory operation when they are applied to hydraulic jacks of conventional design or in other words to jacks in which the piston is fitted with a packing seal. However, precautions must be taken to ensure that the very high overpressures which appear within the damping or "dashpot" chambers are not transmitted to the packing rings of the jack piston since these latter would otherwise become rapidly unserviceable.
This complicates the construction of jacks to an even greater extent, particularly if it is desired to achieve practically perfect reliability in the control of electric circuit-breakers and very long service life without maintenance.
However, these known damping systems of the floating ring type would be inapplicable to differential jacks in which the piston is not provided with a packing ring since they would fail to ensure leak-tight closure of the supply/drain orifice of the main jack chamber at the end of the tripping stroke of the jack. This would result in permanent oil leakage and therefore in permanent oil consumption throughout all periods during which the circuit-breaker is in the open position, which is an unacceptable situation.
The object of the present invention is to overcome these disadvantages and to permit the construction of a differential jack with a ring-type damping system which is of simple design and offers higher reliability of operation than in the past. The invention applies to a jack of the type described in French patent No. 87.04.134 cited earlier, namely in which the piston is combined with a check-valve for shutting-off the supply/drain orifice of the main jack chamber, the closure member of said check-valve being positively actuated by the piston so as to close said orifice at the end of the tripping stroke of said piston, and in which the piston is not provided with any packing ring.